The invention relates to an electron multiplier structure comprising at least one microchannel plate having secondary electron emission, which plate comprises an input face and an output face spaced from the input face. The invention also relates to the manufacture of such a structure and to the use thereof in a photo-electric tube.
It is known that the amplification of a microchannel plate with secondary electron emission, hereinafter abbreviated as M.C.P., is restricted by the saturation as a result of the charge of the walls of each channel during the multiplication. The maximum amplification G.sub.max corresponds to the maximum charge which is obtained at the output of a channel by multiplication of an electron at the input of said channel. This maximum amplification G.sub.max is obtained only when the ratio between the length and the diameter of the channel is sufficiently large. The value of G.sub.max inceases with the diameter of the channels (for example, for d=12.5 .mu.m the maximum amplification G.sub.max is in the order of magnitude of 10.sup.5 and for d=40 .mu.m, G.sub.max is in the order of magnitude of 10.sup.6). In picture display devices in which such channel plates are used, the increase of the amplification by increasing the diameter of the channels is at the expense of the spatial resolving power. Moreover, when such amplifications are used, great problems occur which have to be solved. Amplifications of more than 10.sup.4 can hardly be used in straight channels since with these amplifications the ions formed within the channels form a source of stray phenomena by reaction with the input of the channels, for example, noise pulses, or even in certain cases an uninterrupted noise, generally referred to as "self-generation". When an emissive surface (for example a photocathode) is brought near the input of an M.C.P., the occurrence of these phenomena is considerably stimulated. Therefore the amplification of the tubes of this type (for example picture amplifier tubes) is purposely restricted to comparatively low values (G.ltoreq.10.sup.4). A solution by which one single M.C.P. can operate at high amplifications (&gt;10.sup.4) without "breakdown" occurring as a result of ion reaction consists in that the channels are given a curvature. In the case of straight channels it is necessary in practice to obtain high amplifications (&gt;10.sup.3) to have the disposal of two or even three microchannel plates which are arranged in cascade and form one or two chevrons. However, this causes some of its characteristics to deteriorate as compared with those of one single M.C.P. operating at its maximum amplification. This relates in particular to the instantaneously obtained characteristics (increase of the pulse response which is associated with the length of the channels), the statistic fluctuation of the amplification, the spatial resolving power (the formation of the electronic avalanche effect between input and output), as well as the noise level which generally increases as a function of the number of microchannel plates whether the system operates or does not operate at its maximum amplification. It is furthermore to be noted that the maximum amplification G.sub.max obtained with a multiplier having several M.C.P.'s can hardly be increased by the addition of a further M.C.P. In that case, besides all the above-mentioned characteristics, also the characteristics which relate to a realizable current pulse counting, namely the level N of said signals at the given frequency F or also the frequency F of said signals for a given level N, are found to deteriorate. A disadvantage when one M.C.P. or a combination of M.C.P.'s operates at a high amplification in fact relates to the occurring decrease of the linear amplification dynamic. The average maximum output current I.sub.S max which can be provided by an M.C.P. during linear operation and the amplification G are as a matter of fact a function of the electric voltage V.sub.G applied between the two sides of the M.C.P. These functions are given by the equations I.sub.S max =0.1(V.sub.G /R.sub.G) and G=K V.sub.G.sup..alpha., wherein R.sub.G is the electric resistance between the sides of the M.C.P. and k and .alpha. are constants, .alpha. being large and for L/d=40 is, for example, in the order of magnitude of 10. From this it follows that to each increase by a factor g of the amplification corresponds a reduction by substantially the same factor of the maximum level I.sub.E max of the current to the input of the M.C.P. which can be amplified during linear operation. With a linear amplification of pulsated signals this results in a decrease of the permissible frequency F for a given level N of the pulses, or conversely a reduction of this permissible level N for a given frequency F. What has been said with reference to the current also applies with reference to the quantity of charge which can be provided by an M.C.P. during linear operation. As a matter of fact it is known that the maximum charge which during linear operation can be provided by a given M.C.P. varies according to V.sub.G and is directly proportional to the multiplication surface used. Hence, the higher the amplification, the lower the pulse charge level which is permissible during linear operation.